This invention is generally directed to data concentration in a time division multiplexed system in which time slot shifting is used. This invention is especially, but not exclusively, suited for use in a telecommunication system in which information to be transmitted during a call is transmitted during an assigned time slot as a digital sample of the information.
Telecommunication networks such as supported by the AT&T 5ESS.RTM. switch support a plurality of telephone calls in which the subscribers' information is carried by digital words representing time samples of the information. Such systems utilize a sequence of time slots, such as every 125 microseconds, in which samples of information are represented by digitized words, such as pulse code modulation encoding of the analog information. In order to conserve bandwidth and maximize transmission line efficiencies, a plurality of such digitized samples are combined to form a consecutive sequence of digital words (time slots) carried by communication channels. A concentrator receives a plurality of subscriber channels as inputs and translates these inputs to output channels which are fewer in number than the input channels. The purpose of the concentrator is to maximize efficiency by recognizing that only a percentage of the total number of subscribers will be making concurrent calls at any given time. The purpose of the concentrator is to eliminate unused time slots on its outbound channels by packing the inbound time slots on its outbound channels to maximize available bandwidth.
Such systems typically utilize a time slot interchanger (TSI) to shift the position of a time slot to prevent an over capacity situation which could result when the number of inbound time slots to be transmitted during a given time slot exceed the number of outbound channels on the concentrator The TSI consists of memory sufficient to store the digital words associated with each time slot in a frame and a corresponding control memory corresponding to each time slot in the frame to control which stored time slots are to be transmitted at each time slot position within the frame. Such an implementation provides a maximum of flexibility since each digital word may be shifted to any time slot within the frame. However, such flexibility has the disadvantages of being relatively expensive, requiring substantial memory, and introducing significant delay. Thus, there exists a need for an economical and simplified mechanism, and method, for providing time slot shifting to support concentration.